The present invention relates to a Czerny-Turner spectroscope, and especially to its optical system. Such a spectroscope is used, for example, in an ultraviolet and visible spectrophotometer or other spectrophotometers.
Capillary electrophoresis (CE) is an analyzing method suited for analyzing peptides, proteins, nucleic acids, sugars or other biological substances. It is also suited for use in an optical resolution, isotropic separation or other separation of very close substances at high speed. Thus it is used in clinical diagnoses or in monitoring of various medicines and environmental substances.
However, the capillary has a small outer diameter of about 100 to 400 micrometers, and is fragile even though it is usually protected by a polyimide coating. Therefore, the user must be extremely careful during the process of exchanging it. Furthermore, an accurately measured injection of sample into a capillary is difficult, and on-capillary reaction schemes usually require junctions which are difficult or tedious to make without introducing an extra volume. These have led to the proposal of a capillary electrophoretic chip (referred to as a microchip in this application) formed by connecting two substrates to each other, as described in Anal. Chim. Acta 283(1993), pp.361-366 by D. J. Harrison et al.
FIG. 5 is a perspective view of an exemplary microchip 10 used in such a system. The microchip 10 is composed of a pair of transparent plates (glass plates, quartz plates, or plastic plates) 11, 12 attached together. On the top face of the lower plate 11 are formed two crossing grooves, one for a sample loading channel 13 and the other for a separation channel 14. In the upper plate 12 are formed four through holes respectively located at both ends of the channels 13 and 14, which are used for reservoirs R1, R2, R3 and R4. The channels 13 and 14 are formed by etching the transparent plate 11 (or by any other method), with the width of generally 10-100 micrometers and the depth of generally 5-50 micrometers.
The measurement is performed as follows. First, media liquid is poured from any one of the reservoirs R1-R4 to fill the channels 13 and 14. Then, a small quantity of liquid sample is injected into one of the reservoirs R1 and R2 at an end of the sample loading channel 13, and a high voltage is applied between a pair of electrodes respectively put in the reservoirs R1 and R2. The liquid sample disperses along the sample loading channel 13. Then, another pair of electrodes is put respectively in the two reservoirs R3 and R4 at the ends of the separation channel 14, and a migration voltage is applied between the pair of electrodes. The sample at the crossing of the channels 13 and 14 migrates in the separation channel 14.
FIG. 6 shows a cross sectional view of the microchip 10 cut along the separation channel 14. An ultraviolet and visible spectrophotometer is placed at the end of the separation channel 14 as the detector. The detector is composed of a spectroscope 30 and a photo detector 31 placed across a near end part of the separation channel 14. Light from a lamp (not shown) is introduced into the spectroscope 30, where a monochromatic light of a certain wavelength is extracted and is cast onto the sample migrating in the separation channel 14. The transmitted light (or the reflected light) is received by the photo detector 31 and the strength is measured. Absorbance of the monochromatic light is calculated based on the measured strength and a component or components of the sample are identified.
A Czerny-Turner spectroscope is often used as the spectroscope 30 for a CE. A Czerny-Turner spectroscope includes: a plane diffraction grating; a first concave mirror (collimator mirror) for reflecting the light from the entrance slit to the plane diffraction grating; and a second concave mirror (telemeter mirror) for reflecting the light reflected and separated by the plane diffraction grating to the exit slit. As the plane diffraction grating is rotated about an axis passing through the center of its surface, the wavelength of light passing through the exit slit varies, whereby a wavelength scanning is performed.
In a conventional Czerny-Turner spectroscope, both the collimator mirror and the telemeter mirror have spherical surfaces. With such an optical construction, the shape of a monochromatic light (or a monochromatic component of the spectrum) P obtained from the spectroscope is not straight but curved, as shown in FIG. 7. Thus, the monochromatic spectrum component of a desired wavelength P only partly covers the separation channel 14. In the other part of the separation channel 14, the wavelength of light passing through the separation channel 14 deviates from the desired wavelength, which deteriorates the accuracy of measurement.
An object of the present invention is, therefore, to provide a spectroscope that can produce monochromatic spectrum components of a less-curved or straight form.
The inventors discovered using aspheric (non-spherical) surfaces for the collimator mirror and the telemeter mirror. Through intensive experiments and analyses, we invented a proper construction and configuration of the surfaces of the mirrors. Then, a Czerny-Turner spectroscope according to the present invention includes: a collimator mirror having a toroidal surface for reflecting a beam of light passing through an entrance slit to a plane diffraction grating; and a telemeter mirror having a cylindrical surface for reflecting and converging the beam of light reflected and separated by the plane diffraction grating to an exit.
When a linear spectrum restrictor is placed at the exit, the straight generatrix of the cylindrical surface of the telemeter mirror is set to lie substantially parallel to the linear spectrum restrictor. That means, when a slit is placed at the exit, the straight generatrix is set substantially parallel to the exit slit. When a linear object, such as the separation channel of the microchip, to receive a part of the spectrum of light reflected by the telemeter is placed at the exit, the linear object is set parallel to the straight generatrix.
The above cited Czerny-Turner spectroscope is used in a broader sense which includes a crossed-type Czerny-Turner spectroscope in which the incident beam and the reflected beam of the plane diffraction grating cross and a Czerny-Turner spectroscope in a narrower sense in which they do not cross.
According to the present invention, the curving of monochromatic spectrum component at the exit is minimized. When a linear spectrum restrictor is placed at the exit, the monochromatic light produced by the inventive Czerny-Turner spectroscope can enter the linear spectrum restrictor in full. Thus, when used, for example, as the detector of a microchip electrophoresis system, stability and reproducibility of capillary electrophoresis measurements are achieved. The inventive Czerny-Turner spectroscope can also be used in a spectrophotometer for general analyzers, whereby the accuracy and reliability of the measurements are improved.